1. Field of the Invention
This invention is concerned with transducers for use under great hydrostatic pressures. In particular, it is directed towards reducing the rate of change in power sensitivity of a piezoelectric transducer as a function of increasing pressure.
2. Discussion of the Prior Art
Piezoelectric transducers of various configurations are extensively used is seismic exploration, in acoustic tracking of submarines in the deep ocean and even for recording the conversations of whales. Variations in acoustic pressure produce an electrical output from the transducers, measured in the range of microvolts to millivolts.
For use in hyperbaric environment on the order of 10,000 psi, and for good and sufficient design reasons, piezoelectric transducer elements in the form of a right cylinder are preferred. The right cylinder is sealed at each end by an end plate, leaving an internal air space. Use of such a tubular transducer element, for example, is shown in U.S. Pat. No. 4,162,476 as reference numeral 1 of FIGS. 1 and 4 of the patent drawings. The cylinder is closed at each end by end plates 2 and 3 of the drawings. It is apparent that the axial stress applied to the cylindrical element 1 is equal to the full area of the end plates multiplied by the ambient static pressure (for purposes of this discussion, we shall ignore the presence of the accelerometer element shown in the figures of that reference). It is to be presumed that the static pressure is uniformly distributed around the element.
Another version of a tubular or cylindrical transducer is found in U.S. Pat. No. 3,739,326. Here, the transducer is wrapped around the stress member of a streamer cable and is held in place by end caps. The end caps present an area to axial forces much larger than the full diameter of the transducer element and therefore do not minimize the axial stress relative to the radial stress.
I have found, from laboratory measurements and field tests, that the capacitance, and hence the sensitivity or power output, of commercially available tubular ceramic piezoelectric transducers diminishes non-linearly with increasing water depth or pressure. In an off-the-shelf commercial unit, the drop in power sensitivity was 63% over a range of static pressures from atmospheric to 10,000 psi.
From further laboratory tests, I have found that whereas the capacitance of a tubular ceramic transducer increases with increasing radial stress, its capacitance diminishes with increasing axial stress. My tests show that the rate of change of diminishing capacitance due to axial stress is much greater than the rate of change of increasing capacitance due to radial stress. The net result therefore, of an increasing uniform pressure field around the transducer element is a lowering of its capacitance as explained above.
It is a purpose of this invention to provide a transducer element suitable for use at high ambient pressures, whose rate of change in power sensitivity as a function of pressure is minimized.